In the discussion herein, the term anode defines the electrode at which electrons are taken from the electrolyte solution or bath and the term cathode defines the electrode at which electrons are supplied to said bath. Conductivity of the electrolytic current within the bath is carried out by the simultaneous migration of negatively charged anions to the anode and of positively charged cations to the cathode.
The partial reactions taking place at the respective electrodes in the particular process to which this invention relates can be represented as follows. At the anode, chloride ions and hydroxyl ions give up electrons according to the overall anodic reaction EQU Cl.sup.- + 6 OH.sup.-.fwdarw.ClO.sub.3 .sup.- + 3 H.sub.2 O + 6e
and at the cathode, water molecules accept electrons from the cathode according to the overall cathodic reaction EQU 6 H.sub.2 O + 6e .fwdarw.3 H.sub.2 + 6 OH.sup.-
since there is no diaphragm in the cell, mixing occurs, facilitated naturally by the evolution of hydrogen gas and optionally enhanced by mechanical agitation. Thus the hydroxyl ion formed at the cathode is made readily available for the anodic reaction.
Now both the water and the technical grade of sodium chloride commonly used in industrial manufacture of sodium chlorate almost always contain cations of the alkaline earths, particularly calcium, and also magnesium. These alkaline earths become deposited on the cathode, usually as carbonate if the anode is graphite or substantially as hydroxide when the anode is a metal, in the form of a closely compacted scale which adheres closely to the cathode, tending to insulate the cathode electrically and significantly thus to increase the resistance of the cell, necessitating a substantial increase in the total electrical potential applied across the terminals of the cell in order to maintain a constant electric current. The formation of compactly textured deposit becomes more rapid when the temperature is raised or when the electric current density is increased.
Another harmful effect of alkaline earth cations occurs when the anodes are made of a metal covered with a surface layer of an electrochemically active substance, as exemplarily titanium covered with a layer of ruthenium oxide. The alkaline earth cations can facilitate building up anodic deposits injurious to good functioning of the anode.
Thus the presence of alkaline earth cations in the electrolyte leads on the one hand to an increase in the specific energy consumed and on the other to the necessity of submitting the cathodes and cells to a periodic cleaning. The need for such cleaning is all the more frequent when the working temperature is raised and/or when the electrical current density is increased. This tends to cancel the advantages which accrue from the use of specially coated metal anodes, such as titanium coated with ruthenium oxide. The major interest in using such special electrodes is to permit electrolysis at high temperature and under high current density. However the frequent cleaning action which is then required risks deterioration of the anodes.
The cleaning operation is itself cumbersome, involving stages of stopping the electrolysis, emptying the cell, scouring the anodes, rinsing the cell, reintroducing the electrolyte and starting up again. Inasmuch as this procedure involves work of great magnitude, it is usually considered inadvisable to interrupt cell operation in this manner unless the coating has accumulated to extreme proportions.